The present invention relates to a lithium secondary battery which has extremely high safety as well as high energy density and in particular is preferably used for the drive motor of an electric vehicle.
In recent years, regulation of emissions of carbon dioxide has been highly demanded, with the environment protection movement reaching its heights in the background. The automobile industry, to replace automobiles using fossil fuels, such as vehicles driven by gasoline, is proceeding in earnest with development of motor-drive batteries that will be the key to putting EV into practical use with the aim of promoting the introduction of electric vehicles (EV) and hybrid electric vehicles (HEV).
As a battery for such EV and HEV, in recent years, attention has been paid to lithium secondary batteries with high energy density, which enables them to extend the running distance per charge compared to use of lead storage batteries or nickel hydrogen batteries.
In a lithium secondary battery, a lithium compound is used as a positive active material, while various carbon materials are used as a negative active material, and an organic solvent in which a lithium ion electrolyte is dissolved is used as an electrolyte solution, which constitutes the battery. At charging, lithium ions in the positive active material are transferred from the positive active material to the negative active material in the electrolyte solution, permeating a porous separator separating the positive active material and the negative active material. At discharge, on the other hand, the lithium which was captured by the negative active material is ionized and transferred to the positive active material, thus conducting charging/discharging.
Here, since a lithium secondary battery has a large energy density compared to a conventional secondary battery, strict guidelines are provided with regard to its safety. For example, according to the xe2x80x9cLithium Secondary Battery Safety Assessment Standard Guidelines (commonly called the SBA Guidelines)xe2x80x9d issued by the Battery Association of Japan, a lithium secondary battery is required not to burst, nor catch on fire even if the entire amount of energy that was fully charged to its charging capacity were to be instantly discharged by an external short circuit or an internal short circuit caused by a nail (metal rod) piercing test, etc., giving rise to heat in the battery.
An internal short circuit similar to the above-described nail piercing test, may occur due to the fact that the appropriate quantity of working volume (filling weight) for the electrode active material has not been determined. Practically during the reaction at the time of charging, when lithium ions in the positive active material are captured by the negative active material, if the lithium ions are supplied in a quantity surpassing the lithium holding capacity (the lithium charge/discharge capacity, hereafter referred to as the xe2x80x9ccharge/discharge capacityxe2x80x9d) possessed by the negative active material, metal lithium will be deposited on the surface of the negative active material. Deposition of this metal lithium may then cause dendrite growth, which could result in a short circuit between the positive active material and the negative active material. This dendrite growth is especially apt to occur at the first charging.
When an internal short circuit occurs due to this dendrite, as in the nail piercing test, the energy stored in the negative active material is immediately discharged, resulting in a thermal increase in the battery together with an internal pressure increase. In the worst case, the battery may burst or catch on fire. Since the quantity of energy to be stored in a lithium secondary battery with a large capacity for EV and the like is large, such an internal short circuit could lead to a deadly accident.
On the other hand, it is possible to make the charge/discharge capacity of the negative active material larger than the charge/discharge capacity of the positive active material in order to prevent the occurrence of such dendrite growth by metal lithium. However, unnecessary filling of the negative active material to an extreme extent is not preferred since it may decrease the energy density of the battery.
Accordingly, in theory, equalizing the charge/discharge capacity of the negative active material and the positive active material could make the energy density larger. Therefore, in forming a compact lithium secondary battery for conventional handy electronic equipment and the like, attention has been paid to this point, and design has been conducted by considering the entire working volume ratio of the positive and negative electrodes. Incidentally, the working volume ratio is the ratio of the positive and negative active materials defined as the value that is the working volume of the positive active material divided by the working volume of the negative active material.
However, a large capacity lithium secondary battery for EV and the like formed based on such a design principle leads to the problem that the rate of faulty product may be excessive. Regarding this problem, the present inventors thought that in a large capacity lithium secondary battery, the working volume ratio of the positive and negative active materials should be set to a sufficiently safe value so that dendrite growth of lithium metal would not occur across the entire charging/discharging region where the positive and negative electrodes face the other since the electrodes area is extremely large compared to a conventional compact lithium secondary battery.
That is, it is also thought to be of necessity to consider partial dispersion of the working volume ratios of the positive and negative active materials in each positive and negative electrode within a certain battery, namely the distribution of dispersion of the partial working volume ratios at each positive and negative electrode, such as spotted thickness of electrode active material layers and looseness/tightness of the filling state.
The present invention was achieved by considering the problems of the prior art mentioned above. That is, according to the present invention, there is provided a lithium secondary battery, comprising: a battery case, and an internal electrode body contained in the battery case and including a positive electrode, a negative electrode and a separator made of porous polymer, the positive electrode and the negative electrode being wound or laminated so that the positive electrode and negative electrode are not brought into direct contact with each other via the separator, wherein a working volume ratio of the positive active material and the negative active material obtained by the positive active material weight being divided by the negative active material weight is within the range from 40% to 90% of the theoretical working volume ratio.
According to the present invention, there is further provided a lithium secondary battery comprising: a battery case, and an internal electrode body contained in the battery case and including a positive electrode, a negative electrode and a separator made of porous polymer, the positive electrode and the negative electrode being wound or laminated so that the positive electrode and negative electrode are not brought into direct contact with each other via the separator, wherein the minimum value of the working volume ratio of the positive and negative active materials is regarded as an experimental safe working volume ratio X3 when the usage rate of the negative electrode is 100%, and the electrode areas of the positive and negative electrodes are divided into n elements respectively, yielding a ratio of the average value of working volume weight of the positive and negative active materials at the elements to be regarded as an average working volume ratio Xav while the working volume weight of the positive and negative active materials constitutes a normal distribution of dispersion "sgr", and, the faulty-rate of batteries using the elements is expressed with an upper probability Q(u) (where u=(X3xe2x88x92Xav)/"sgr") of the normal distribution, then, nxQ(u) being the faulty rate of the lithium secondary battery is set to be 1 ppm or less.
In such a lithium secondary battery of the present invention, the difference between the average working volume ratio Xav and the experimental safe working volume ratio X3 is preferably no less than 6 times the dispersion "sgr", and further preferably no less than 6 times and no more than 20 times the dispersion "sgr". In addition, where the ratio of the positive and negative active material weights when the charge/discharge capacity of the positive and negative materials is equal is regarded as the theoretical working volume ratio X1, the difference between the average working volume ratio Xav and the theoretical working volume ratio X1 is preferably no less than 11 times the dispersion "sgr" and further preferably no less than 11 times and no more than 30 times the dispersion "sgr".
Such a lithium secondary battery of the present invention is preferably adopted to a battery whose battery capacitance is no less than 5 Ah, and is preferably used for an electric vehicle or a hybrid electric vehicle.